<Nonaqueous Electrolyte 1 and Nonaqueous-Electrolyte Secondary Battery 1>
With the recent trend toward size reduction in electronic appliances, secondary batteries are increasingly required to have a higher capacity. Attention is hence focused on lithium secondary batteries (nonaqueous-electrolyte secondary batteries), which have a higher energy density than nickel-cadmium batteries and nickel-hydrogen batteries.
The electrolytes used in lithium secondary batteries are nonaqueous electrolytes prepared by dissolving an electrolyte such as LiPF6, LiBF4, LiClO4, LiCF3SO3, LiAsF6, LiN(CF3SO2)2, or LiCF3(CF2)3SO3 in a nonaqueous solvent such as a cyclic carbonate, e.g., ethylene carbonate or propylene carbonate, an acyclic carbonate, e.g., dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate, a cyclic ester, e.g., γ-butyrolactone or γ-valerolactone, an acyclic ester, e.g., methyl acetate or methyl propionate, or the like.
First, various investigations have been made on nonaqueous solvents and electrolytes in order to improve the battery characteristics including load characteristics, cycle characteristics, and storability of such lithium secondary batteries. For example, patent document 1 includes a statement to the effect that when an electrolyte containing a vinylethylene carbonate compound is used, the decomposition of this electrolyte is minimized and a battery excellent in storability and cycle characteristics can be fabricated. Patent document 2 includes a statement to the effect that when an electrolyte containing propanesultone is used, recovery capacity after storage can be increased.
However, incorporation of such compounds has had a problem that although the incorporation has the effect of improving storability and cycle characteristics to some degree, a coating film having high resistance is formed on the negative-electrode side and this, in particular, reduces discharge load characteristics.
<Nonaqueous Electrolyte 2 and Nonaqueous-Electrolyte Secondary Battery 2>
Secondary, various investigations have been made on nonaqueous solvents and electrolytes for use in those nonaqueous electrolytes in order to improve the battery characteristics including load characteristics, cycle characteristics, and storability of those lithium secondary batteries. For example, use of a nonaqueous solvent having a higher permittivity and a lower coefficient of viscosity has various advantages, e.g., the resistance of the electrolyte can be reduced to a low value, as described in non-patent document 1. Furthermore, that nonaqueous solvent is thought to be capable of improving infiltration into the positive and negative electrodes. Use of that nonaqueous solvent is hence preferred.
However, solvents having a heteroelement-containing functional group (group constituting a framework) other than a carbonyl framework, such as ether compounds and nitrile compounds, which are one kind of preferred solvents from those standpoints have the following drawback. These solvents are electrochemically decomposed by an oxidation reaction at the positive electrode or by a reduction reaction at the negative electrode and are hence difficult to use. Practically, carbonic esters or carboxylic acid esters, such as enumerated above as examples, are used in combination. These solvents have a carbonyl group and have excellent oxidation resistance/reduction resistance.
On the other hand, patent document 1 includes a statement to the effect that when an electrolyte containing a vinylethylene carbonate compound is used, the decomposition of this electrolyte is minimized and a battery excellent in storability and cycle characteristics can be fabricated. Patent document 2 includes a statement to the effect that when an electrolyte containing propanesultone is used, recovery capacity after storage can be increased.
However, incorporation of such compounds has had the following problem although the incorporation has the effect of improving storability and cycle characteristics to some degree. When these compounds are used in order to sufficiently improve characteristics, a coating film having high resistance is formed on the negative-electrode side and this, in particular, reduces discharge load characteristics. Especially when those solvents which have a heteroelement-containing functional group (group constituting a framework) other than a carbonyl framework and have a high permittivity and a low viscosity are used, there has been a problem that the preferred characteristics are not imparted.
The desire for higher performances in nonaqueous-electrolyte secondary batteries is growing more and more, and it is desired to attain various characteristics including high capacity, high-temperature storability, continuous-charge characteristics, and cycle characteristics on a high level.
<Nonaqueous Electrolyte 3 and Nonaqueous-Electrolyte Secondary Battery 3>
Thirdly, various investigations have been made on nonaqueous solvents and electrolytes in order to improve the battery characteristics including load characteristics, cycle characteristics, and storability of such lithium secondary batteries. For example, patent document 3 includes a statement to the effect that when an electrolyte containing a phosphinic acid ester is used, a battery inhibited from deteriorating in battery performance during high-temperature storage or during continuous discharge can be fabricated. Patent document 4 proposes a secondary battery which has an excellent life in charge/discharge cycling at a voltage exceeding 4.2V and is fabricated using an electrolyte containing an organic compound having two or more cyano groups.
Especially when a battery is in the state of being continuously charged in which a slight current is permitted to always flow therethrough to keep the battery in a charged state in order to compensate for the self-discharge of the battery, then the electrodes are always in the state of having high activity. Because of this, the battery is apt to suffer accelerated deterioration in capacity or gas evolution is apt to occur due to the decomposition of the electrolyte. In particular, in the case of a battery having high capacity, there is a problem that since the space within this battery has a small volume, the internal pressure of the battery increases considerably even when a slight amount of a gas is evolved due to the decomposition of the electrolyte. With respect to continuous-charge characteristics, not only reduced capacity deterioration but also the inhibition of gas evolution are strongly desired.
However, the electrolytes containing the compounds described in patent document 3 and patent document 4 have been insufficient in the inhibition of gas evolution during continuous charge and in the inhibition of battery characteristics deterioration, although the electrolytes have the effect of improving cycle characteristics and storability to some degree.
<Nonaqueous Electrolyte 4 and Nonaqueous-Electrolyte Secondary Battery 4>
Fourthly, various investigations have been made on nonaqueous solvents and electrolytes in order to improve the battery characteristics including load characteristics, cycle characteristics, and storability of such nonaqueous-electrolyte batteries or to enhance the safety of such batteries during heating or at the time of short-circuiting. For example, sulfolane combines a high permittivity and high electrochemical oxidation stability even in nonaqueous solvents and a boiling point as high as 278° C., which is higher than those of ethylene carbonate and propylene carbonate. Sulfolane can hence be expected to contribute to an improvement in battery safety when used as a solvent. However, sulfolane has a melting point as high as 28° C. and there has been a problem that a battery employing sulfolane as a main solvent has impaired low-temperature characteristics. Furthermore, it is known that sulfolane has poor compatibility with graphite-based negative electrodes and that use of sulfolane as a main solvent results in a charge/discharge capacity lower than a theoretical capacity.
For example, patent document 5 discloses that in a nonaqueous-electrolyte secondary battery employing the electrolyte described therein, the electrolyte can be prevented from solidifying at low temperatures by using a mixed solvent composed of sulfolane and ethyl methyl carbonate.
Patent document 6 discloses that when sulfolane and γ-butyrolactone are used as main solvents and vinylethylene carbonate and vinylene carbonate are added thereto, then a coating film of satisfactory quality which has high lithium ion permeability is formed on the surface of the graphite-based negative electrode and an improved initial charge/discharge efficiency is obtained.
<Nonaqueous Electrolyte 5 and Nonaqueous-Electrolyte Secondary Battery 5>
Fifthly, many reports have been made on the addition of various additives to electrolytes for the purpose of improving initial capacity, rate characteristics, cycle characteristics, high-temperature storability, low-temperature characteristics, continuous-charge characteristics, self-discharge characteristics, overcharge-preventive properties, etc. For example, to add 1,4,8,11-tetraazacyclotetradecane has been reported as a technique for improving cycle characteristics (see patent document 7).
However, the desire for higher performances in nonaqueous-electrolyte secondary batteries is growing more and more, and it is desired to attain various characteristics including high capacity, high-temperature storability, continuous-charge characteristics, and cycle characteristics on a high level. For example, the prior-art technique disclosed in patent document 7, which is regarded therein as effective in improving cycle characteristics, has had a problem that this technique, when used alone, results in considerable gas evolution during continuous charge and in a considerable decrease in recovery capacity after a test, as will be shown later in a Reference Example.
<Nonaqueous Electrolyte 6 and Nonaqueous-Electrolyte Secondary Battery 6>
Sixthly, various investigations have been made on nonaqueous solvents and electrolytes in order to improve the battery characteristics including load characteristics, cycle characteristics, and storability of such lithium secondary batteries. For example, patent document 1 includes a statement to the effect that when an electrolyte containing a vinylethylene carbonate compound is used, the decomposition of this electrolyte is minimized and a battery excellent in storability and cycle characteristics can be fabricated. Patent document 2 includes a statement to the effect that when an electrolyte containing propanesultone is used, recovery capacity after storage can be increased.
However, incorporation of such compounds has had a problem that although the incorporation has the effect of improving storability and cycle characteristics to some degree, a coating film having high resistance is formed on the negative-electrode side and this, in particular, reduces discharge load characteristics.    Patent Document 1: JP-A-2001-006729    Patent Document 2: JP-A-10-050342    Patent Document 3: JP-A-2004-363077    Patent Document 4: JP-A-7-176322    Patent Document 5: JP-A-2000-012078    Patent Document 6: JP-A-2004-296389    Patent Document 7: JP-A-9-245832    Non-Patent Document 1: Kikan Kagaku Sōsetsu, No. 49, p. 108